The present invention relates to a switching mode power supply. More particularly, the present invention relates to the magnetic amplifier of the switching mode power supply.
A magnetic amplifier is typically used in switching mode power supply to act as a high efficiency post-regulator. Most multi-output PWM (pulse width modulation) power supplies can ensure only one master output under the precision regulation, in which the master output is under the feedback loop control. The rest of the outputs are open loop and may be regulated by passive devices such as common-mode inductor, etc. Therefore, a post regulator is needed if another accuracy output is required. The magnetic amplifier (MA) is a high efficiency post regulator for step-down (buck) power conversion. The output voltage of the MA is thus lower than the output voltage of the master output. Please refer to xe2x80x9cAbraham I. Pressman xe2x80x9cSwitching Power Supply Designxe2x80x9d McGraw-Hill Book Co., p381-p412 for the theory and operation of MA. FIG. 1 shows a circuit schematic of the PWM power supply that includes a MA. The PWM circuit controls the master output, and the MA circuit controls the MA output. FIG. 2 displays the waveform of the circuit shown in FIG. 1. Through the feedback loop of the MA, the error-amplifier U1 and transistor Q1 control the current Ib that flows in to the choke L1.
The magnetic-flux density of the choke L1 is biased to Bb by the current Ib, wherein Bb=0.4**N*Ib/lm. The choke L1 plays a role as an on/off switch. As long as Bb existed, the choke L1 is in off-state. After a voltage is applied to choke L1 for a period of time Tz, Bb will be reset to zero and choke L1 will change to on-state.
Tz=Bb*N*Ae/Vp;
N: turn numbers of L1; Ae: core area of L1; Im: magnetic path length of L1;
The MA output voltage Vom is
Vom=Vp*Ton/T;
Where Ton=Tpxe2x88x92Tz and Tp is the pulse width of the PWM signal Vp. A minimum pulse width Tp is required for the MA to fit the specification. The pulse width Tp is controlled by the master feedback loop and is determined by the load condition of the master output Vol. In the case for Vol light load and Vom maximum load, the pulse width Tp might be too short to provide the regulated Vom. Therefore, a dummy load or a consumption of minimum load is needed in the master output Vol. However, the minimum load requirement does not meet the criteria of power management. Power management is a solution for saving power consumption of electronic equipment. Neither the minimum load nor the dummy load is a good approach. No load condition is common in a system that has power management, and the dummy load will decline the efficiency of the power conversion. For this reason, the magnetic amplifier is a high efficiency and low cost regulator, but it could be a good solution only if the minimum load requirement of the power supply can be reduced.
This invention provides a method and apparatus for a magnetic amplifier to reduce the minimum load requirement of power supply. The apparatus comprises a voltage-dropper and a current-limiter. Through the voltage-dropper and current-limiter, the master output of the power supply is coupled to the output of the magnetic amplifier. The differential voltage of the master output and the magnetic amplifier output decide the voltage of the voltage-dropper. The current-limiter limits the current flowing from the master output to the magnetic amplifier output. The voltage-dropper is connected in series with the current-limiter. Through the voltage-dropper and current-limiter, the master output of the power supply is linked to the output of the magnetic amplifier. The feedback loop of the master output controls the pulse width of PWM signal to regulate the master output. Through the regulation of the magnetic amplifier, the PWM signal produces another regulated output, such as the output of the magnetic amplifier. The master output is in no load condition and the magnetic amplifier is in full load condition. A start-up current will flow from the master output to the output of the magnetic amplifier via the voltage-dropper and the current-limiter. In the mean time, the start-up current will widen the pulse width of PWM signal until the current-limiter limits the current. The maximum current of the current-limiter is assigned to maintain the minimum pulse width of the PWM signal. Besides, the current-limiter provides over-load protection for the voltage-dropper. This minimum pulse width of the PWM signal ensures that the output of the magnetic amplifier is inside the regulation.
Advantageously, the method and apparatus for magnetic amplifier can reduce the minimum load requirement of power supply.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.